More than one-third of the US population is now obese. Obesity is associated with colon cancer and other malignancies, and the increased prevalence of obesity threatens to set back advances made in cancer control. It has been proposed that this association could be due to growth- promoting effects of hyperinsulinemia. We propose an alternative in which hyperinsulinemia and cancer risk is both mediated by insulin resistance. We have previously published a striking phenotype of mice with endothelial cell knockout of the insulin receptor gene (Insr), an important aspect of endothelial dysfunction in obese humans. These animals have 4-fold increased leukocyte adhesion to endothelial cells because of endothelial cell upregulation of vascular cell adhesion molecule-1 (VCAM-1). Preliminary data from my lab now show that tumor-prone Apc (Min/+) mice with endothelial cell Insr knockout (IRKO/M mice) have increased tumor numbers in the small intestine compared with controls. They also have increased immune cell infiltration in small intestinal tissue. We therefore hypothesize that obesity-associated insulin resistance in endothelial cells promotes immune cell recruitment to the gut which in turn promote tumor development. With Aim 1, we will quantitate endothelial cell rolling and adhesion to endothelium in intestinal venules using in vivo microscopy. We will also determine whether treatment with an alpha4 integrin inhibitor, which blocks binding of leukocytes to VCAM-1, prevents the accelerated tumorigenesis in IRKO/M mice. With Aim 2, we will use Apc(Min/+) mice with transgenic overexpression of insulin receptor substrate-1 (IRS-1) in endothelial cells (Tg/M mice) or their controls (wt/M) after feeding them a high-fat or control diet. We will determine whether tumor burden and lekocyte rolling/adhesion in intestinal wall venules in vivo are improved in obese Tg/M mice compared to obese wt/M mice. The genetically modified mice described for both aims are available in our laboratory. This proposal is highly innovative because insulin resistance in cancer target tissues has not previously been thought to be the cause of the association between hyperinsulinemia and cancer risk in obesity. In addition, we introduce mouse tumor models with loss and gain of insulin signaling targeted to endothelial cells which allows us to study insulin resistance independent from hyperinsulinemia, thereby overcoming the problem that these two factors cannot be separated in humans or most animal models. The proposed research is significant because it will provide a target pathway for prevention of cancer in obesity.